From Storm in Heaven:

Last year, Taichi nearly died on a snowy mountain, but was saved by his best friend, Shinobu. The only problem is that Taichi doesn't remember a thing about this so-called childhood friend of his! Assuming his memories will return, Taichi keeps this little secret to himself until, a year later, he suddenly finds himself with a much bigger secret. You see, Shinobu is more than he seems, and Taichi might not have actually survived that night on the snowy mountain!